Field of the Invention
The present invention relates to a dynamically stable, passive, totally magnetically energized bearing/suspension system that does not require eclectically activated servo controlled systems to attain a stable equilibrium at operating speed, and more specifically, it relates to means for adjusting passive magnetic bearing stiffness.
Description of Related Art
U.S. Pat. No. 5,495,221 describes an invention that achieves a state of stable equilibrium above a critical speed by use of a collection of passive elements using permanent magnets to provide their magneto-motive excitation. The magnetic forces exerted by these elements, when taken together, levitate the rotating object in equilibrium against external forces, such as the force of gravity or forces arising from accelerations. At the same time, this equilibrium is made stable against displacements of the rotating object from its equilibrium position by using combinations of elements that possess force derivatives of such magnitudes and signs that they can satisfy the conditions required for a rotating body to be stably supported by a magnetic bearing system over a finite range of those displacements. This prior invention can be described as a magnetic bearing system containing at least two discrete subsystems, at least one of which is energized by permanent-magnet material. (More typically, three to four subsystems would be employed). These subsystems, when properly disposed geometrically, act together to support a rotating element in a state of dynamic equilibrium. However, owing to the limitations imposed by Earnshaw's Theorem, the magnetic bearing systems do not possess a stable equilibrium at zero rotational speed. Therefore, means were provided to hold the suspended system in equilibrium until its speed has exceeded a low critical speed where dynamic effects take over and thereby permit the achievement of a stable equilibrium for the rotating object.
It is desirable to provide techniques for adjusting the magnitude and/or reversing the sign of the stiffness of the passive bearing at small gaps. The present invention provides such techniques.